This invention relates generally to liquid crystal-on-silicon (LCOS) imagers.
LCOS imagers are increasingly being applied in high definition rear projection televisions, as well as front projectors. One important characteristic of LCOS imagers is the contrast ratio, which is the ratio between the on and off reflectance. In order to provide a good contrast ratio, the liquid crystal cells induce a relative phase retardation of π between the switched states of the reflected light. This may be achieved via electric field induced rotation of the liquid crystal molecules in the cell.
However, the molecules in the layers adjacent to the electrodes undergo incomplete rotation due to anchoring on the alignment layers, resulting residual birefringence that lowers the contrast. In such cases, an external birefringent plate may be attached to the panel to compensate for the inadequate phase retardation.
Currently, such compensators may be based on stretched polymer films sandwiched between two glass plates. These stretched polymer film compensators induce birefringence by stretching the polymer films. However, such birefringence may be non-uniform, resulting in a non-uniform dark state. Also, the stretch induced birefringence may degrade over time as the film relaxes. In addition, particles trapped in the sandwich may create defects in the dark state. As a result, the stretched film compensators have not been amenable to volume manufacturing and result in part-to-part variations.
Another existing compensator is based on a single crystalline thin plate that is difficult to grow in a large area and, hence, expensive.
Thus, there is a need for compensators to improve the contrast ratio in LCOS display imagers.